A combined cycle power plant can use a combination of a gas turbine and a steam turbine to produce electrical power. In a combined cycle power plant, a gas turbine cycle can be operatively combined with a steam turbine cycle by way of a heat recovery steam generator (“HRSG”).
In a combined cycle power plant, the gas turbine cycle can be referred to as a topping cycle, and the steam turbine cycle can be referred to as a steam bottoming cycle. Since the steam turbine or bottoming cycle is driven by heat from the exhaust of the gas turbine or topping cycle, the HRSG does not, in some instances, become fully operational until the gas turbine or topping cycle has increased the steam turbine or bottoming cycle to a suitable temperature.
For example, during start-up of the gas turbine or topping cycle, there is a relatively rapid increase in the flow rate of the hot gas exhaust from the gas turbine as the turbine accelerates to operating speed. At this point, the temperature of the exhaust gas gradually increases as the firing temperature of the gas turbine is increased and managed at a suitable level to produce a desired power output.
Although the hot exhaust gas from the gas turbine typically flows through the HRSG during the gas turbine start-up, a considerable period of time can elapse before an initially cold HRSG is capable of generating steam at sufficient pressure and temperature. In conventional systems, the gas turbine or topping cycle was kept at a relatively low load until the temperature of the HRSG increased to a level where the HRSG could generate steam at a desired pressure and temperature. By maintaining the topping cycle at a low load for an extended period of time, the steam passing to the steam turbine could be controlled at a temperature and pressure that would reduce stresses on the cold steam turbine metal and component parts. When the topping cycle was not maintained at a low load for this warm-up phase, the steam turbine or bottoming cycle was subjected to stresses that reduced its operational life. Parts fatigue, casing and shaft distortions, and physical deterioration of the HRSG system's seals and blades are just some examples of the damage these stresses can cause. In contrast, operating the gas turbine or topping cycle at a relatively low load can reduce these stresses and corresponding damage. Doing so, however, can reduce the combined cycle system's overall power output, leads to inefficiency, and increases emissions.
Consequently, there is a need for systems and methods for pre-warming a HRSG and associated steam lines. Furthermore, there is a need for systems and methods for pre-warming a HRSG and associated steam lines in multiple state conditions, such as cold, warm, and hot conditions.